Evidence has been presented that some aspects of allograft tolerance can be explained by the presence of serum factors blocking the specific lymphocytes that react against cells carrying "tolerant" antigens. Using the microcytotoxicity test, it is possible to analyze vertical changes in immune responses in rats, dogs and humans. The purpose of the present study is to: a) analyze the changes in immune response during the course of induction of neonatal tolerance in rats; b) analyze the changes in the immune response during and after termination of the tolerant state; c) analyze the changes in the immune response during and after the transfer of tolerance to the normal host; d) correlate the in vitro results with the in vivo status of a tolerated allograft; e) analyze the effect of blocking serum factors on induction of graft vs. host and on immune parameters; f) provide specific non-reactivity to allograft (kidney) antigens in dogs (to circumvent the danger of immunosuppression therapy) and analyze the role of serum factors in such dogs; g) characterize the molecular nature of the blocking serum factors which mediate immunological non-reactivity; h) compare in vitro data obtained by different techniques and correlate this data with in vivo status of allograft. The future application of such studies lies in revealing the mechanism of tolerance and its possible relationship to immunological enhancement (in both organ allografts and tumor rejection).